The present invention is a method for analyzing silicon for nonmetallic contaminants. The method consisting essentially of:
(A) forming an alloy comprising silicon and a metal which promotes separation of nonmetallic contaminants present in the alloy, PA1 (B) separating the nonmetallic contaminants from the alloy, and PA1 (C) analyzing the separated nonmetallic contaminates for chemical content. PA1 (A) forming an alloy comprising silicon and a metal which promotes separation of nonmetallic contaminants present in the alloy, PA1 (B) separating the nonmetallic contaminants from the alloy, and PA1 (C) analyzing the separated nonmetallic contaminants for chemical content. PA1 (A) forming an alloy comprising silicon and a metal which promotes separation of nonmetallic contaminants present in the alloy, PA1 (B) separating the nonmetallic contaminants from the alloy, and PA1 (C) analyzing the separated nonmetallic contaminants for chemical content.
The present invention is especially useful for analyzing metallurgical grade silicon intended for use in the direct process for the production of organohalosilanes for the presence of oxides and carbides of calcium, aluminum, and silicon.
Organohalosilanes, particularly dimethyldichlorosilane, are important intermediates in the silicone industry. The organohalosilanes are typically hydrolyzed and condensed to form polyorganosiloxanes which can then be processed to form, for example, silicone fluids, elastomers, and resins. The predominant commercial process for preparing these organohalosilane intermediates is one commonly referred to as the "direct process," as originally described by Rochow, U.S. Pat. No. 2,380,995 issued Aug. 7, 1945, and Rochow et al., U.S. Pat. No. 2,380,995, issued Aug. 7, 1945.
Because of the high volume of organohalosilanes used in the silicone industry, considerable effort has been devoted to optimizing the conversion of the silicon to the organohalosilanes, particularly diorganodihalosilanes. It is known in the silicone industry that different lots of metallurgical grade silicon react differently in the direct process.
To attempt to control the lot-to-lot variability of the reactivity of metallurgical grade silicon in the, direct process, manufacturers of organohalosilanes have set strict controls on the acceptable types and levels of contaminants present in the silicon. Clarke, J., Organometallic Chemistry, 376:165-222 (1989), provides a comprehensive review of the direct process for synthesis of methylchlorosilanes and the effects of contaminants on the process.
The present inventors believe that a significant cause of the lot-to-lot variability in the reactivity of metallurgical grade silicon in the direct process is the presence of nonmetallic contaminants in the silicon, such as oxides and carbides of calcium, aluminum, and silicon. While these oxides and carbides are believed detrimental to the direct process, low levels of certain metallic species containing these metals are considered to be beneficial to the direct process.
Standard methods for analyzing metallurgical grade silicon for contaminants involves an elemental analysis of the bulk of the silicon for elements such as calcium and aluminum. Therefore, these methods of analysis do not distinguish between, for example, calcium and aluminum which may be present in the metallurgical grade silicon as metallic species and calcium and aluminum which may be present in the silicon in a detrimental nonmetallic form.
An objective of the present invention is to provide a method where nonmetallic forms of elements, such as calcium, aluminum, and silicon can be distinguished from the other forms in silicon. The inventors have found that alloying the silicon with a metal which promotes separation of nonmetallic contaminants present in the alloy provides a method where the nonmetallic contaminants can be separated from the alloy and analyzed by standard methods for their elemental contents.